S.... XJED DAMAGE-RISK CRITERION FOR IMPULSE NO!SE (GUNFIRE) (U)

W. Dixon Ward, Editot

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NAS-NRC Committee on Hearing, Bioacoustics, abd Blomechanics

. -) Working Group 57

AD 673 223

PROPOSED DAMAGE-RISK CRITERION FOR IMPULSE NOISE(GUNFIRE)

W. Dixon Ward

National Academy of Sciences-National Research CouncilWashington, D. C.

July 1968

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Report of Working Group 57Proposed Damage-Risk Criterionfor Impulse Noise (Gunfire)NAS-NRC Committee on Hearing,Bioacouatics, and Biomechanics,1968.

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NATIONAL ACADEMY OF SCIENCES - NATIONAL RESEARCH COUNCILCOMMITTEE ON HEARING, BIOACOUSTICS, AND BIOMECHANICS

PROPOSED DAMAGE-RISK CRITERION FOR IMPULSE NOISE (GUNrIRE) (U)

Report of Working Group 57

W. Dixon Ward, Chairman and Editor

Sg. Cdr. R.R.A. Coles David HodgeLt. Col. John Fletcher Michel LoebGeorges Garinther James D. MillerJ, Donald Harris Horace 0. Parrack

Office of Naval Research Reproduction in whole or inContract No. NONR 2300(o5) part is permitted for any

purpose of the United StatesGovernment.

July 1968

Contents

Page

I. Introduction ........................................... 1

II. Proposed Criteria ...................................... 2

Definitions ......................................... 2

Basic Criterion ...................................... 2

Correction Factcrs ................................... 5

III. Explanation and Justification .......................... 5

IV. Limitations ............................................ 6

References ................................................... 8

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PROPOSED DAMAGE-RISK CRITYERION FOR IMPULSE NOISE (GUNFIRE)

I. Introduction

In 1964, the Committee on Hearing, Bioacoustics and Biomechanicsof the National Research Council (CHABA) proposed a set of damage-riskcriteria (DRC) for intermittent and continuous steady-state noise (Kryter,Ward, Miller and Eldredge, 1966). These criteria were based on the as-sumption that the permanent hearing losses (noise-induced permanent thres-hold shift, or NIPTS) eventually produced by many years of exposure tonoise is approximately equal to the auditory fatigue (temporary thresholdshift, or ITS) shown by a normal ear after a single day's exposure. Acorollary of this assumption is that exposures which produce equal TTSswill produce equal NIPTSs. Therefore, in order to derive DRC for a widerange of exposures, it was merely necessary to select a value of TTS thatshould not be exceeded, and then determine from a study of the literaturewhat noise exposures (expressed in terms of level, duration, and rate ofinterruption) produced precisely this TTS. The CHABA curves cited abcvewere based on the assumption that the acceptable values of TTS 2 (TTS mea-sured two minutes after cessation of exposure to the aoise) were 10 dB at1000 Hz and below, 15 dB at 2000 Hz, or 20 dB at 3000 Hz or above.

Unfortunately, at that time little information on TTS producedby impulse noise existed, aud even this was somewhat ambiguous. It wastherefore not judged possible to estimate what pattern of impulse-noiseexposure would produce, in the average person, the TTSs cited above. Theonly specific statement in the CHABA proposal regarding impulse noise wastherefore the following: "While exact limits cannot be set, the WorkingGroup did find evidence that repeated exposure to some types of acousticimpulses exceeding 140 dB in the earcanal of the listener can result insignificant losses of hearing in some persons."

In the intervening period, several studies at laboratories bothhere and in England have dealt with a fairly large range of exposure togunfire under controlled conditions. These recently were summarized byColes, Garintner, Hodge and Rice (1968), who then proceeded to recommenda DRC for impulse noise based on these data, a DRC designed to protectseventy-five percent of the men exposed. The following proposal is pat-terned closely after the Coles et al criteria; however, the permittedvalues here are slightly different from theirs, for reasons cited inSection III.

II. Proposed Criteria

Definitions

Impulse noises are broken down by Coles et al into two generaltypes, illustrated in figure 1, though intermediate forms do occur. Figurela shows the pressure waveform that is often observed when a gun is firedoutdoors with no reflecting surfaces nearby, while figure lb exemplifiesa much more complicated situation: an initial series of damped oscillationswhich may be followed by a reflected wave at only a slightly lower level.The following terms must be defined, in order to specify the DRC for thesetwo types of impulse noise.

(1) The peak pressure level (P) is the highest instantaneouspressure level reached at any time by the impulse, expressed in decibelsre 0.0002 dyn/cm2 , measured at the position of the ear with the individualnot present.

(2) Te pressure-wave duration, or A-duration, is the time re-

quired for the initial or principal wave to reach the peak pressure leveland return momentarily to zero. In figure la, this duration is from pointV to point W.

(3) The pressure-envelope duration, or B-duration, is the totaltime that the envelope of the pressure fluctuations (positive and negative)is within 20 dB of the peak pressure level, including reflected waves. Thusin figure lb, the B-duration would be from V to X, plus Y to Z. The special-ized measurement techniques required for tcurate determinations of P, A-duration and B-duration are discussed in detail by Coles et al.

Basic Criterion

Figure 2 presents the fundamental criterion, a criterion intendedto limit the TTS 2 produced in all but the most susceptible five percent ofexposed individuals to the CHABA limits of TTS. This DRC represents thetolerance limits for 100 impulses distributed over a period of four minutesto several hours on any single day. IZ is assumed that the pulses reachthe ear at normal incidence. In case of doubt as to which waveform analysisto apply, the more conservative B-duration should be used. The main featuresof the criterion are these:

(1) The maximum peak pressure level permitted is 164 dB (withoutear protection) for the shortest pulses of any practical interest (25 micro--seconds).

(2) As duration increases, the permitted peak pressure level de-creases steadily at a rate of 2 dB for each doubling of the duration, droppingto a terminal level of 138 dB for B-durations of 200 to 1000 milliseconds.

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(3) A similar decrease occurs for A-duratio':5. except that aterminal level of 152 dB is reached at about 1.5 mil!.:•>conds.

Correction Factors

In case the conditions stipulated for this 'asic criterion arenot met, correction factors can be applied as folloi-:

(1) If the pulses arrive at the ear at gr-.z--ng incidence in-stead of normally, the curves can be shifted upward 5 dB (that is, 5 dBcan be added to the ordinate values in figure 2).

(2j If the number of pulses in an "exposr-:c period" (that is,

on any given day) is some value other than 100, an a.justment is made

according to the curve in figure 3. This curve pr. ;ides a 5 dB changein permitted level for each 10-fold change in numb-r of impulses.

III. Explanation and Justification

The Lbasic DRC of figure 2 is essentially i0 dB lower than theone proposed by Coles et al. Half of this difference (5 db) representsthe difference between the two proposals as to whf:ther direction of in-cidence of the impulses is assumed to be grazing )r normal. Coles et aldefine their basic criterion in terms of grazing incidence, with a 5 dBdecrease in allowable limits if the impulse arrives normally, while thepresent prcposal does just the opposite, establishing limits for normalincidence, with a 5 dB increase in permitted level if arrival is grazing.

The other 5 dB by which the present criterio% is more conserva-tive than that of Coles et al stems from the fact that an attempt has beenmade here to protect ninety-five percent oi the exposed personnel insteadof oev;enty-five percent.

In the main, then, the basic criterion is not inconsistent withthat of Coles tc al. However, three changes are more substantive.

(1) At the high end, the termination of the basic DRC at 164 dBmeans that under no conditions should any ear be exposed to a peak levelin excess of 179 dB which is the limit for a single pulse (+10 dB) at graz-ing incidence (+5 dB) with a 25 microsecond duration. Furthermore, theDRC is a straight line in contrast to Celes et al, whose criterion curvesupward for very short pulses. These changes are consistent with some datagathered by Loeb and Fletcher (1968) after Coles et al had prepared theirproposal. Loeb and Fletcher found that 30 dB of TTS 2 was produced in themedian listener by one hundred 30 microsecond pulses whose peak level was167 dB. Since Coles et al had at hand little data on TTS from pulsesshorter than 200 microseconds on which to base their estimate, relyinginstead on some judgments of relative loudness of various pulses, the pre.sent criterion is considered more realistic.

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(2) There is a "floor" of 138 dE for B-durations of 200 to 1000milliseconds. This boundary reflects :he fact that because of reflex con-traction of the middle-ear muscles, the effect of acoustic energy enteringthe ear later than 100 to 200 milliseconds after onset of the oulse willbe considerably reduced.

(3) A specific correction for number of pulses is established,as portrayed in figure 3. It will be noted ti-at since a 10-fcld changein number of irpulses changes the DRC by only 5 dB instead of 10 dE, thiscorrection factor is merely an interpolation and extension of a consideredopinion expressed by Coles et al as follows: "Where exposure is to occas-ional single impulses only, it seems reasonable to raise the limits some-what, and an estimate of 10 dB has teen agreed upon for this."

IV. Limitations

While these curves do no great violence to the published dataon either TTS or PTS from impulse noise (as Coles, Garinther, Hodge andRice show in some detail in the unabridged 1967 version of their recom-mendations) they admittedly represent only a first attempt at a reasonableDRC for impulses. Parameters that are ignored in the present criterionmay eventually be shown to be important. For example, the rise time ofthe initial or principal pulse is not considered here, nor is the periodof oscillation of an "B-duration' impulse. In addition, taking the effec-tive B-duration to be the time needed to drop 20 dB from the principalpeak pressare is rather arbitrary.

Futhermore, the 138-OB, aAd 152-dB plateaus are only grossestimates. The correction for number of impulses, coo, is based on verylimited data. Fin3lly, this criterion, like the earlier CHABA DRC forcontinuous noise, rests heavily on the assumption of a consistent relationbetween TTS and FTS, and this may after all be incorrect.

It is expected, however, that this criterion designates reasonablelimits for the type of impulse noise to which most service personnel will lieexposed: rifle and pistol reports from his own and his fellows' weapons, andsingle rounds fired by higher-caliber armament in both reverberant and non-reverberant conditions. Even automatic weapon fire shouk. probably be cov-ered, if one considers each burst as a single "event". (The justificationfor this, once again, comes from the protective action of the middle-earmuscles, which provides some 10 to 20 dB of effective protection againstall but the first round or two in the burst).

Thus if exposures are limited to the levels proposed here, eitherby control of the peak level or the number of pulses, or by use of adequateear defenders, fewer than the most susceptible five percent of the exposedpersonnel will demonstrate temporary changes in auditory sensitivity solarge that,if they were to become permanent, they would constitute begin-ning auditory handicap.

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It is intended that the limits for impulse-noise exposure out-lined in this document should apply as often as operational and/or safetyconditions permit. For instance, ear protectors snould always be worn onfiring ranges and on most occasions of field firing exercises and otherforms of weapon training, practice, or proving. It is not intended toimply that ear protection should be worn when actually in combat, exceptwhere weapons are i'red from positions out of the immediate zone of fight-ing. Even in the immediate zone of fighting ear protection may sometimesbe advantageous. The effective loss of hearing produced by use of an earprotector can be quickly eliminated by removal of the protector; on theother hand, the loss of hearing produced by the action of the noise onthe unprotected ear requires many hours before recovery to normal hearingsensitivity occurs.

Not only is it necessary to p:otect personnel from eventualpermanent threshold shifts, but for many operational situations it isimperative to protect against TTS, since sensitive hearing is often es-sential for patrol and sentry duty, or in tasks where auditory communica-tion is critical. (In this respect, it is also necessary to protect theears against TTS induced by non-impulse noise sources, such as helicop-ters and armered pers.onnel carriers).

REFERENCES

Coles, R. R. A., G. R. Garinther, D. C. Hodge and C. G. Rice, "Criteriafor Assessing Hearing Damage Risk from Impulse-Noise Exposure,"Technical Memorandum 13-67, U. S. Army Engineering Laboratories,10-erdeen Proving Ground, Maryland 21005

Coles, R. R. A., G. R. Garinther, D. C. Hodge and C. G. Rice, ">-zardousExposure to Impulse Noise," J. Acoust. Soc. Am. 43, 336-343 (1968).

Kryter, K. D., W. D. Ward, J. D. Miller and D. H. Eldredge, "HazardousExposure to Intermittent and Steady-State Noise, " J. Acoust. Soc.Am. 39, 451-464 (1966).

Loeb, M. and j. L. Fletcher, "Impulse Duration and TTS." Report from Ex-perimental Psychology Division, U. S. Army Medical Research Labora-tory, Fort Knox, Kentucky 40121. (Submitted for publication in J.Acoust. Soc. Am.)

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DOCUMENT CONTROL DATA. R & D

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?roposed Damage-risk Criterion for Impulse Noise (Gunfiie) (U)

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Hearing ProtectionImpulse NoisePermaient Hearing LossTemporary Hearing LossDamage-risk CriteriaSafetyDeafness

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